1. Field of the Invention
The present invention relates to an image forming apparatus used with copying machines, laser beam printers and the like, and, more particularly, it relates to an image forming apparatus including an image bearing member (referred to as "photosensitive drum" hereinafter) and a transfer roller.
2. Related Background Art
In a conventional image forming apparatus including a photosensitive drum or exposure drum and a transfer roller, for example, as shown in FIGS. 10 and 11, a transfer roller 26 made of elastic material is pressed against a photosensitive drum 22 by a spring 6, and a toner image formed on the photosensitive drum 22 is transferred to a recording sheet carried by the transfer roller 26 by applying a bias voltage to the transfer roller 26. In this case, a portion 26b of the transfer roller 26 (referred to as "contacting portion" hereinafter) which is being pressed against the photosensitive drum 22 will be gradually deformed and be decreased in its diameter, with the result that the peripheral speed of the contacting portion 26b of the transfer roller 26 will be slower than that of the other portions of the transfer roller 26.
In order to avoid such inconvenience, as disclosed in the Japanese Patent Laid-Open No. 56-126872, transfer roller 26 was driven independently of the photosensitive drum 22 so that the peripheral speed of the contacting portion 26b was equal to that of the photosensitive drum 22. More particularly, transfer roller 26 was driven by a discrete driving means different from the driving means for driving the photosensitive drum 22, in such a manner that the peripheral speed of a point situated on the contacting portion 26b of the transfer roller 26, which has the smallest diameter, was the same as the peripheral speed of the photosensitive drum 22 and the peripheral speed of the other portion of the transfer roller 26 was higher than that of the photosensitive drum 22.
So long as either photosensitive drum 22 or transfer roller 26 is subjected to a driving force, the other can be driven by the same driving force. In this case, as shown in FIG. 14, even if a gear 17 of the photosensitive drum 22 is not actually engaged with a driving gear 16 (of the driving means for driving the photosensitive drum 22), the photosensitive drum 22 is rotated by a rotational force F.sub.T (FIG. 13) caused by contacting transfer roller 26.
On the other hand, when the photosensitive drum 22 and transfer roller 26 are drivingly rotated by driving both of these elements 22, 26 by means of respective driving means, so that the peripheral speed of the portion, having the smallest diameter, of the contacting portion 26b of the transfer roller 26 is higher than the peripheral speed of the photosensitive drum 22, as shown in FIG. 15, the transfer roller 26 is subjected to a driving force f.sub.2 from the driving means through a driving gear 14 (of the driving means) and a gear 15 (of the transfer roller) and to a resistance force F.sub.D for resisting the rotation of the transfer roller 26 caused by the photosensitive drum 22. While the photosensitive drum 22 is subjected to the rotational force F.sub.T tending to accelerate the rotation of the photosensitive drum 22 caused by the transfer roller 26, and at the same time, is subjected to a rotational force f.sub.3 tending to decelerate the rotation of the photosensitive drum 22 from the associated driving means through gears 16 and 17. In this case, when the driving force f.sub.3 is not transmitted to the photosensitive drum 22 from the driving gear 16 of the driving means (i.e., a tooth of the gear 16 is not engaged by a tooth of the gear 17 or the photosensitive drum is subjected to the force tending to resist the rotation thereof), if a leading edge of a recording sheet S is struck against a fixing device (not shown), a shock or impact force will be transmitted to the photosensitive drum 22 and transfer roller 26 through the recording sheet S, thereby returning the photosensitive drum 22 to a position shown by a dotted line (FIG. 15). Then the tooth of gear 17 of the photosensitive drum 22 is contacted to the tooth of driving gear 16, thus stopping or decelerating the photosensitive drum 22 temporarily. Thereafter, photosensitive drum 22 is rotated at a normal speed by the driving force f.sub.1 (FIG. 12) from the driving gear 16. Accordingly, in such a case, there arises a problem that an image transferred to the transfer roller 26 is distorted or sheared due to the uneven rotation of the photosensitive drum 22. Such problem will also arise in the case where a sheet conveying path is curved downstream of the image transfer position, since the sheet is subjected to a variable friction force by such a curved path.